Vision testing using eye charts has long been used in evaluating an individual's vision. Early charts were printed on paper or other like material, displayed vertically on a wall or other support and illuminated by the ambient light within the room. Because the light in the room was produced primarily by the sun, the light intensity varied significantly with the season and the weather conditions. In turn, since the human eye perceives shapes better under increased lighting conditions, it can be appreciated that variations in the amount of light illuminating the eye chart would have a significant effect on the eye test results.
In the early testing rooms, the light striking the eye chart was dependent on both the light provided from within the room as well as sunlight entering from outside the room. To decrease the variation in illumination, testing rooms were made windowless to allow only the artificial light to illuminate the eye chart. This light was more controllable by the tester, and thus served to decrease variability and thereby improve the reproducibility of results over time. An alternative method of conducting the vision test was to print the test onto translucent glass and provide illumination from behind.
Though illumination of the test chart by the use of only artificial light improved the reproducibility of the test method, variations still existed. The light bulbs used to create the artificial light did not produce a uniform light output. As the bulbs aged, their output intensity varied. This non-linear output hampered the efforts of eye testers to compare test results obtained over a period of time.
One device which attempted to address the illumination variation problem was produced by Vistech Consultants, Inc. A prototype unit was introduced in 1985, and was ultimately designated the MCT-8000. The unit provided front lighting onto an eye chart surface and had a photocell located adjacent to the light bulb for detecting the bulb output. The photocell was connected to a readout device. The readout was manually compared to factory-determined light intensity values which would correspond to day- and night-time lighting conditions.